Aqueous dispersions of tetrafluoroethylene polymers which contain glass beads or glass bubbles are known to be useful for coating onto glass fabric to provide a film thereon to protect and strengthen the glass fabric. The beads or bubbles aid in filling the fabric weave openings, and help provide protection to the fabric. Thus Cook U.S. Pat. No. 3,928,703 describes the use of glass beads or hollow glass bubbles in such dispersions for coating onto glass fabric.
The stability of the beads or bubbles in such a dispersion has long been of concern. German OS No. 2,315,259 describes the use of organic thixotropic thickeners to aid in keeping glass beads or bubbles in suspension; while Ribbans U.S. Pat. No. 3,380,770 describes the use of selected ionic electrolytes to aid in maintaining glass beads in suspension. Thickening is expected to retard the sinking or flotation of a filler with a density different from that of the medium in which the filler is dispersed. This is clear from Strokes" Law which describes the motion of a body under gravity, falling or rising as the case may be, and which specifically states that the limiting velocity should vary inversely with the viscosity of the medium. Nevertheless, thickening within a useful range of viscosity is not expected to prevent the sinking of a high density filler or the flotation of a low density filler. Thickened dispersions to the Ribbans patent exhibit settling of the filler during the normal commercial interval between production and use. The OS referred to above teaches that, even with thickening, the density of the glass bubbles should be approximately the same as that of the dispersion in which it is used. This would avoid the anticipated limit on the benefit of thickening by completely eliminating buoyancy, or the tendency to float. However, commercially available glass bubbles have very low density, approaching or even equaling the density of cork, and consequently are extremely buoyant.
It would be desirable to provide a dispersion containing glass bubbles which has both good stability with respect to flotation of the bubbles and a viscosity in a range to permit good coating onto fabric. Such a dispersion is provided by this invention.